Positive ion electrostatic accelerator



Sept. 11, 1956 cm. TURNER 2,762,941

POSITIVE ION ELECTROSTATIC ACCELERATOR Filed March 22, 1955 2 Sheets-Sheet 1 01/ INVENTOR.

CLARENCE M. TURNER Sept. 11, 1956 c. M. TURNER 2,762,941

POSITIVE ION ELECTROSTATIC ACCELERATOR Fi led March 22, 1955 2 Sheets-Sheet 2 ELECTRON ABSORBER OF LOW ATOMIC NUMBER 4 4 r I I)! 9e 2 I56 w q ELECTRON ABSORBER OF LOW ATOMIC NUMBER INVENTOR. CLARENCE M. TURNER POSITIVE ION ELECTROSTATIC ACCELERATOR Clarence M. Turner, Stony Brook, N. Y., assignor to the United States of America as represented by the United States Atomic Energy Commission Application March 22, 1955, Serial No. 496,083

21 Claims. (Cl. 313-63) This invention relates to an improved method and apparatus for accelerating positively charged ions. The invention is particularly directed toward the improvement ol the performance characteristics of electrostatic generators, used to accelerate positive ions, by suppressing the generation of X-rays which has been inherent in the operation of such accelerators. In the operation of these machines, the appearance of the X-rays was accompanied by a current drain on the system which increased rapidly with increasing voltage or ion beam current. This current drain reduced in substantial measure the voltage that could be accumulated on the high-voltage terminal.

The phenomenon of X-ray generation in apparatus of this type has long been observed and various attempts have been made to minimize its adverse effects. For example, in my patent, Number 2,578,908, an improved vacuum system yielding a higher vacuum in the accelerating tube of such a machine is described as effective in suppressing this phenomenon. While this does improve the situation somewhat, the improvement is relatively slight, and until my present invention, the expedients tried hardly began to provide a solution to the problem.

This invention is based on my discovery that the adverse etfect's of the X-rays are the result of the ionization by the X-rays of the high-pressure insulating gas used as an atmosphere around the high-voltage terminal of the accelerator to reduce the tendency for flashover. In conventional electrostatic generators, such as the Van de Graaff type, unless some measures are taken to limit the X-ray flux in the high-field regions filled with the highpressure insulating gas, the current drain resulting from the ionization can reach a magnitude of several hundred microamperes, comparable to the total charging current available for maintaining the voltage of the high-voltage electrode. This current drain is caused by the formation of large quantities of ions in the gas, and the negative ions formed are precipitated onto the high-voltage (positive) electrode. Furthermore, some of the negative ions, as much as 20-30% of those formed, also reach and are deposited on the hoops that are part of the system for maintaining the field gradient along the accelerating tube, thereby disturbing the uniformity of the field gradient. Since these X-rays are caused, at least in large measure, by electrons which stream back along the interior of the accelerating tube toward the high-voltage electrode, this disturbance of the voltage gradient may easily establish a regenerative process: The ionization and precipitation of the negative ions can distort the voltage gradient in such a way that the electron stream may be further increased, in this way generating a higher flux of X-rays and thereby still further increasing the ionization.

My discovery that these deleterious elfects are the results of X-rays ionizing the high-pressure insulating gas around the high-voltage terminal has now led to a re markably simple, yet very effective solution to the problem. As an example of the efficacy of my solution, I was able to increase the maximum energy attainable on one 2,762,941 Patented Sept. 11, 1956 electrostatic generator from 2.25 m. e. v. to 3.8 m. e. v. In fact before my improvement the threshold for the appearance of the X-ray phenomenon described above was 1 m. e. v.

Based on my discovery, this invention, broadly speaking, resides in improving the performance characteristics of an electrostatic-generator type of positive-ion accelerator by surrounding the generator with an atmosphere of insulating gas under pressure and suppressing the ionization thereof by X-rays generated by back-streaming electrons in the generator, thereby to maintain the insulating effect of the atmosphere. Two techniques that I have found valuable in eliminating the effect of the X-rays are to inhibit the generation of the X-rays and to absorb X-rays that may be produced. A very effective method of inhibiting the generation of X-rays is to cause the back-streaming electrons to strike a material containing at least one element having an atomic number less than 13, for example, beryllium, carbon, boron or magnesium, or compounds such as lithium hydride or boron carbide. Any X-rays that are generated are eflfectively absorbed in a material of high density, such as lead, bismuth, uranium and other radiation shielding materials. These materials should be electrically conductive so that they do not accumulate a charge and thereby interfere with the operation of the accelerator. When the material has a low electrical conductivity, it may be coated with an electrically conductive material, for example, by evaporating a film of aluminum on its surface. To accomplish their purposes these materials are interposed in the stream of electrons or X-rays at strategic positions hereinafter more fully described.

The objects, advantages and some of the modifications of this invention are described hereinafter with reference to the following drawings in which- Figure 1 is a full sectional view of an electrostatic generator used for accelerating positive ions and showing the general arrangement and relationship of the components;

Figure 2 is a cross section, somewht diagrammatic, of the ion source and the high voltage end of the acceleration tube of an electrostatic generator of the type shown in Figure 1 and illustrates an embodiment of this invention; and

Figure 3 is a diagrammatic cross-section of a portion of the ion-accelerating tube showing a modification of the invention.

While Figure 1 illustrates the major components of a positive-ion accelerator of the electrostatic-generator type, additional details of construction and operation may be had upon reference to my Patent Number 2,578,908, issued December 18, 1951.

With' reference first to Figure 1, the electrostatic generator has a cylindrical tank 10 with a horizontally oriented axis and dishedends. The tank 10 has a fixed and a horizontally movable part removably joined at a flanged seam 12 and gasketed there to form a fluid tight vessel. Within the fixed portion of the tank 10, there is a removably vertical face plate 20 which carries the principal interior generator elements on an arrangement of cantilevers.

At the end of tank 10 opposite the face plate there are arranged the group of elements that provide the high potential. The high-voltage electrode itself, designated by reference numeral 22, is a hollow metal surface of revolution supported from the face plate 20 by cantilevers 24 fabricated of an insulating material such as tubing sold under the name Herkolite or the like. The cantilevers 24 are encircled internally and externally by nu merous rings 26, 28 which are part of the system used to distribute potential in a uniform gradient. More par-' ticular reference will be made to this system hereinafter. For rigidity, the cantilevers 24 are braced with spaced plates 30, 32 and 34 to which the cantilevers are aifixed. The stificning plates 30, 32 and 34 are pierced with suitable apertures for the passage of the charging belt 36.

The endless charging belt 36, which is composed of a flexible insulating material such as cotton canvas or the like, enters the high voltage electrode 22 through suitable openings in the end facing the face plate 20. At the other end of the tank, the charging belt 36 passes through suitable openings in face plate 29. An idler pulley '38, located just within the high-voltage electrode, and a drive pulley 4t), adjustably mounted on the distal side of face plate 29, support the belt. A suitable electric motor 42 within the tank drives pulley 40 and thus'the charging belt 36.

To establish the necessary high positive potential, a high-voltage rectifier 44 furnishes a moderately high potential to a series of needle points 46 supported above the pulley 40. The corona discharge from the row of needles points 46 toward the pulley 4t) sprays-a static charge onto the surface of the intervening belt as it travels from pulley 40 to high-voltage electrode 22. Where it enters the high-voltage electrode, the static charge on belt 36 is transferred to a second series of needle points 48 electrically connected to the idler pulley 38. Since pulley 38 is electrically insulated from the high-voltage electrode, it becomes positively charged and this charge is transferred by corona discharge across an adjustable gap 50 to the high-voltage electrode. As the belt leaves the high-voltage electrode it passes across a third row of needle points 52, electrically connected to gap 50 and to the high-voltage electrode. With'pulley 38 continuously maintained at a positive potential with respect to the high-voltage electrode, negative charge is sprayed on the belt by needle points 52. Thus, as described more fully in my patent referred to before, the belt serves to transport positive charge to the high-voltage electrode and to transport negative charge from it. Thereby a high positive potential is established on the highvoltage electrode with respect to the grounded face plate.

Means are provided to distribute this potential in a uniform gradient along the accelerating path. This type of system will be described but briefly here; a fuller explanation and description will be found :in my patent designated hereinbefore.

The potential gradient between the'high-voltageelectrode and the grounded face plate is governed by an assembly comprising a tube 54 of an insulating material, extending between the face plate 20 and the highvoltage electrode 22 and parallel to the charging belt 36. At its high-voltage-electrode end, the tube 54 is sealed and at the face-plate end, the tube 54 is connected by a pipe, containing a throttle valve 60, to asupply 64 of compressed nitrogen.

The voltage gradient along the tube 54 is determined by a corona discharge device comprising metal discs 68 positioned at close regular intervals. Each of the metal discs 68 carries a corona discharge needle pointed toward the high-voltage electrode and closely spaced from the next adjacent disc. When ahigh potential has been established, a corona discharge takes place between the need e points and discs across the .small gaps there-between. In this way current flows from the high-voltage electrode and definite increments of potential drop are thereby maintained between adjacent discs. By adjusting the nitrogen pressure in tube 54 the current carried may be varied. The system serves very effectively as a row of resistors connected in series.

The voltage-gradient steps established on the discs 68 are used to regulate the voltage distribution on other surfaces. Conductive rings 26 and 28, spaced similarly to the discs 68, encircle the interior and exterior, respectively, of each of the tubular cantilevers 24 and are electrically connected to the discs 68. Then the correspond- 2 ing pairs of rings on the cantilevers are interconnected by large, conductive hoops 74 encircling the entire canti lever structure to define planes of constant potential. Similarly, rings 76, electrically connected into the gradient system encircle the upper and lower spans of the belt 36 to provide a uniform gradient along the belt.

To suppress ionization and flashover to the tank wall, the tank is filled with gas whose essential characteristic is that it tends to trap free electrons. A convenient gas for this purpose is arnixture of carbon dioxide and nitrogen, the latter being a diluent. As shown in Figure l, a bank of cylinders or gas bottles 84 containing a -compressed dry mixture of nitrogen and 20% carbon dioxide communicates with a common manifold 78. The compressed gas mixture flows from the manifold into the tank 10 through a pipe 86 containing a throttle valve 89. A pressure in tank 10 of the order of 200 pounds per square inch or less is deemed sufiicient to prevent flashover at potentials as high or higher than four million volts. The carbon dioxide-nitrogen mixture may also be formed by burning natural gas, propane or other hydrocarbon gases in air and drying the product, as described in my patent number 2,578,908. Air itself may be used, but introduces a corrosion problem and fire hazard. Freon or sulfur hexafluoride have also been used with nitrogen as a diluent.

Positive ions generated in the ion source located within the high-voltage electrode 22 are accelerated in the accelerating tube 83 supported between the high-voltage electrode and the face plate 20. The accelerating tube, whose high-voltage end is shown in greater detail in Figure 2, is composed of many annular corrugated ceramic sections 90, formed for example of zircon porcelain or like material. The ceramic annuli are separated by thin washer-like metal annuli 93, which are shown more clearly in Figure 2. The metal annuli 93 extend outward beyond the surface of the ceramic sections 90, and each is electrically connected to a coplanar member of the system of potential planes. The annuli 93'also extend into the accelerating tube and there support'and make contact by a sort of spring action with electrodes 92. Each of the electrodes 92, of an annular shape generally formed by spinning, has a bowl-like wall dished in the direction of the ion current and an internally di' rected lip or flange 96. An outwardly directed curved lip 94 around the periphery ofthe electrode engages a corresponding metal annulus 93 and the wall of the accelerating tube to space the electrodes precisely.

The accelerating tube is connected through the wall of tank 10 to a vacuum manifold 98 which is exhaustedby suitable pumping equipment to maintain a pressure in the'accelerating tube of 10 mm. of mercury or less. The ion 'beam emerges from the tube through a suitable aperture 99 in the wall of the vacuum manifold.

The ion-source assembly, designated by reference numeral 100, is located at the high-voltage end oft-he accelerating tube 88 with-in the high-voltage electrode.

The ion source 162 is of a conventional arc type. The source comprises two reservoirs 104 for the ioncharge gas, one each for hydrogen'and'deuterium gas. Each of the reservoirs is served by its own palladium regulator 106 and feed conduit 110 containing a control valve 108. The feed conduits communicate through a manifold 112 with the cathode chamber of the ion source. 'Eac'h palladium regulator 106 is provided with a heating element 11410 take advantage of the property of palladium to become increasingly permeable to hydrogen or deuterium with increasing temperature. Thus the flow of gas to the ion source is regulated by regulating the current flowing through the heating element.

By adverting now to Figure 2, the ion source and some associated apparatus can be describedin some detail, The ion source is mounted by means of an annular flange 122 on the end'ring 124 of the accelerating tube 88. The source is divided into an ion-forming arc chamber 126 and an ion-steering and accelerating chamber 128. The chamber 128 is in a cylindrical compartment 129 coaxial with the accelerating tube 88. Compartment 129 is formed of an annular insulator 150 held between the flange 122 and a connecting plate 152. The arc chamber 126 is in a separate compartment 116 mounted in a central opening in the connecting plate 152.

The are chamber 126 is in turn divided into a cathode chamber 118 and an anode chamber 120 connected by a narrow aperture or neck 122 through which the arc is struck. The ion-forming gas is admitted to the cathode chamber through inlet pipe 112. The neck between the cathode and anode concentrates the arc and at the same time provides a relatively high concentration of the ionforming gas at the point of maximum arc intensity.

Power for the arc is supplied by apparatus within the high-voltage electrode, as indicated in Figure 1. The source of the power is a permanent-magnet generator 132 whose armature is rotated by a belt driven by idler pulley 38. The alternating, three-phase voltage generated is converted to direct current by a conventional three-phase bridge rectifier 134 whose output, regulated by the arc control circuit 136, is applied across the anode and cathode of the arc. Arc control is provided by a variable resistor.

After forming the positive ions, they are withdrawn from the arc chamber, focussed into a beam and started through the accelerating tube. With reference again to Figure 2, a probe 138 having a conical nose is spaced by an aluminum oxide ring-insulator 142 from the surface 'If a conical recess in the wall of the arc chamber. A capillary passage 140 in the nose of the probe is alined with the aperture 130 and a probe shield 144 is provided to shield the ion beam from stray charges that may collect on the insulator 150. As indicated in Figure 1, a variable source 146 of potential, of the order of 25 kilovolts, is connected between the arc chamber and the probe. When this is energized, the ions are drawn from the are through the probe.

The ion beam thus withdrawn from the arc passes through an accelerating gap and then is guided into the desired path by a pair of steering plates. With reference to Figure 2, a flanged, hollow, cylindro-conical accelerating electrode 156 is mounted coaxially with the accelerating tube 88 in the central opening of flange 122. The small gap 161 is maintained between the probe 138 and the frusto-conical, open end of the electrode 156 which is alined with the probe aperture 140. A source of potential 146 is connected to the electrode 156 to maintain its potential at a value higher than that of the probe. The potential difference thereby established across the gap 161 imparts an initial acceleration to the ion beam. Parallel steering plates 148 are mounted by insulating posts 158 on the shield 156 and extend into it a considerable distance toward the probe. The source 154 of potential and a beam steering circuit 159 are, as shown in Figure 1, connected between the steering plates 148 and the arc compartment 116.

With the foregoing picture in mind, we may turn briefly to the operation of an electrostatic-generator type of linear accelerator to describe what has been a vexacious problem prior to the present invention. After a period of operation of the charging belt 36, a high positive potential is established on the high voltage electrode with respect to the faceplate. Under the influence of the potential, ions generated in the ion source 100 are accelerated along the accelerating tube 88 and a large proportion emerge from the aperture 99.

It was found that as the voltage was increased on the high-voltage electrode, the machine behaved normally up to a certain threshold voltage. Then, quite suddenly, X-rays began to appear and simultaneously there was an increasing current drain on the system. As the voltage was increased further, the X-ray flux and current drain both increased very rapidly. Numerous tests were performed to locat'e'the current drain and to eliminate it. In the course of the tests it was learned that the threshold voltage marking the appearance of the X-rays was sensitive to the pressure of the residual gas in acceleration tube 88. The evidence gathered seemed to indicate that the effect was due entirely to electrons streaming back through the accelerating tube from the low to the high potential end.

The electrons may originate in one or more of several ways. It will be remembered that the ions are collimated by the ion source and further focussed by the electrodes 92. Nevertheless some of the ions strike the walls and other parts of the accelerating tube, the vacuum piping and the residue of gas remaining under the vacuum conditions in the accelerating tube, and the impingement of the ions produces electrons in the accelerating tube. Secondary electrons may also be produced when the primary electrons strike the materials of the tube wall and electrodes. Still other electrons may be produced by the Malter effect in which electrons are emitted from underlying metallic surfaces under the influence of a high positive charge built up on films of oil or of other insulating materials. There may also be other sources of electrons. From whatever origin, under the high potential difference existing along the accelerating tube, the electrons flow in a current toward the high-voltage electrode. In the belief that this back-streaming electron current was the current drain on the machine, the value of the electron current was carefully measured. It did not account for the observed value of the current drain on the system. In fact, elaborate measurements of all of the possible leakage paths did not account for the observed value. i

I have discovered that the current drain is not the emission of electrons in the accelerating tube but rather the emission is only a causative agency in a process that comprises several steps. The electrons streaming back in the accelerating tube strike the various parts of the apparatus, and particularly the ion source. This results in the emission of X-rays which travel into the insulating gas filling the tank 10. The X-rays in turn ionize the gas to produce negative (and positive) ions. The negative ions, in turn, drift back to the high-voltage electrode and to the positively charged voltage-gradient system and discharge a part of the potential thereon. The process has a multiplicative effect: one electron accelerated toward the high-voltage electrode may produce several ions in the insulating gas. Thus it is that the electron current streaming back in the accelerating tube did not account for the observed current drain.

To suppress this eflect, in accordance with my invention, the electrons are absorbed in a material of low atomic number. A plate 160, of beryllium preferably, is mounted over the face of the ion source so as to intercept the electrons streaming back through the accelerating tube 88. The diameter of the plate is slightly larger than the central opening in the electrodes 92. The plate 161) has a small central aperture 162 alined with the aperture of the ion source probe 138 to permit the passage of the ion beam.

The elements of low atomic number have the singular property of emitting relatively few X-rays when absorbing electrons, and of them all beryllium is the most efiicacious for it has a high melting point and chemical stability upon exposure to atmospheric humidity. Lithium too is suitable provided suitable precautions are taken to avoid contact with the atmosphere and to keep it cooled below its melting point. The electron absorber should be electrically conducive to avoid accumulating a charge.

It is desirable to absorb even the few X-rays that are emitted from the electron absorber. A radiation-absorbing shield 164 of lead or other heavy metal is located between the electron-absorbing plate and the ion source. The shield 1.64 is in the form of .acup surrounding plate 160 on three sides and open toward the accelerating tube 88. An aperture 166 in the base of the cup is alined with the aperture 162 in the electron-absorbing plate and the probe aperture 140 to permit the passage of the ion beam. The wall of the cup is carefully fitted against the first electrode 92. In this manner, substantially all the relatively small flux of X-rays emitted from the electron-absorbing plate may be absorbed since practically none of the X-ray emission is in the direction of the ion beam, but almost all the emission is toward the ion source and at an angle of 90 or less.

Still further protection against the X-ray phenomenon is furnished by maintaining a potential, eithernegative or positive, on the plate 160 relative to the X-ray absorber 164. This serves to reduce the electron current striking plate 160 and concomitantly to suppress still further the production of X-rays. The reduction in the electron current by this means is thought to occur in this way: When electrons (and possibly negative ions .as well) strike the plate 160, positive ions are formed from the contaminants which almost invariably are found on or in the material of plate 160. These positive ions strike the parts of the acceleration tube and strike the residual gas therein to liberate additional electrons. However, when there is a potential difierence between plate 160 and cup 164, there is established in the immediate vicinity of the plate a region of non-uniform field. This non-uniformity imparts a radial component to the positive ions leaving the plate so that they are directed away from the axis of the accelerating tube. In this way, the positive ions are eliminated before traversing any considerable length of the accelerating tube and being accelerated :to high energies thereby.

The non-uniformity in the field in the vicinity of plate 160 is most conveniently accomplished by connecting plate 160 electrically to a power supply. The plate 160 is spaced from the shield 164 by insulating posts 168. A conductor 170 is connected at one end to plate 160 and at the other end to a high-voltage terminal 172 which is in turn connected to a source of 'high potential 173 (shown on Figure l) or to the high-voltage electrode. The conductor passes through a suitable opening in the lead shield and is insulated therefrom.

It may be desirable in certain instances to furnish still further protection against this effect. Under certain combinations of the design parameters of the acceleration tube 88, a considerable flux of Xrays is produced by the collisions of electrons with the electrodes 92. The plate 160 has little value in suppressing the generation of X-rays from this source. However, the necessary additional protection may be furnished by rings 174 of an element of low atomic number, preferably beryllium, atfixed on the inwardly directed lips 96 of electrodes 92, as shown in Figure 3. The rings 174 are brazed or otherwise fastened to the surface of each lip facing away from the high-voltage electrode. The internal diameter of the rings is slightly smaller than that of the electrodes to shield their edges from electron bombardment. In this modification, the electrodes 92 may be fabricated of a dense material to absorb any X-rays that may result from electron capture in the rings 174.

Still better control of the X-ray generation may be achieved by fabricating the electrodes 92 of a material of low atomic number, again preferably beryllium. The X-ray absorbing material, containing a dense element, may be incorporated in the annular insulating sections 90 and metal annuli 93 of the acceleration tube 88. Other alternatives will be apparent to those skilled in the art.

Since many embodiments may be made of this invention and since many variations will be evident to those skilled in the art, the foregoing is to be interpreted as illustrative only. The invention is to be measured only by the scope of the claims following hereafter.

I claim:

, l. The method of improving the performance characteristics of an electrostatic generator employed for accelcrating positive ions including surrounding said generator with a body of insulating gas under pressure and maintaining the insulating effect of said gas by inhibiting the flux of ,X-rays in said gas which are generated by backstreaming electrons striking generator surfaces, thereby suppressing the ionization of said gas.

2. The method of improving the performance characteristics .of an electrostatic generator employed for accelerating-positive ions including surrounding said generator with a body of insulating gas under pressure and maintaining the insulating effect of said gas by suppressing the ionization thereof by X-rays generated by back-streaming electrons within said generator, including effecting said suppression by absorbing the X-rays generated by said electrons striking against generator surfaces.

3. The method of improving the performance characteristics of an electrostatic generator employed for accelerating positive ions including surrounding said generator with .a body of insulating gas under pressure and maintaining the insulating eflect of said gas by suppressing the ionization thereof by X-rays generated by backstreaming electrons within said generator, including effecting said suppression by absorbing said electrons in a material having an atomic number less than 13.

4. The method of improving the performance characteristics of an electrostatic generator employed for accelerating positive ions including surrounding said generator with .a body of insulating gas under pressure and maintaining the insulating effect of said gas by suppressing the ionization thereof by X-rays generated by backstream'ing electrons within said generator, including effecting said suppression by absorbing said electrons in a material having an atomic number less than 13 and by ab-- sorbing any X-rays resulting from the electrons striking against said'material.

5. 'In an electrostatic generator for positive-ion acceleration comprising a fluid-tight tank, an ion-accelerating tube in said tank, means for establishing a voltage gradient along said tube and an ion source at the high-voltage end of said tube, the improvement comprising an inert insulating gas atmosphere filling said tank under superatmospheric pressure and means for absorbing electrons streaming back toward the high-voltage end whereby the generation of X-rays and the consequent ionization of the gas are suppressed.

6. The apparatus of claim 5 in which the gas contains as its only active ingredient a material that traps free electrons.

7. .In an electrostatic generator for positive-ion acceleration comprising a fluid-tight tank, an ion-accelerating tube in said tank, means for establishing a voltage gradient along said tubeand an ion source at the high-voltage end of said tube, the improvement comprising an inert insulating gas atmosphere filling said tank under superatmospheric pressure and means within said accelerating tube for absorbing electrons streaming back toward the highvoltage end whereby the generation of X-rays and the consequent ionization of the gas are suppressed.

8. The apparatus of claim 7 in which the electronabsorbing means is an electrically conductive material of low atomic number.

9. The apparatus of claim 7 in which the electronabsorbing means is fabricated of an electrically conductive material comprising an element of atomic number lower than 13.

10. in an electrostatic generator for positive-ion acceleration comprising a fluid-tight tank, an ion-accelerating tube in said tank, means for establishing a voltage gradiout along said tube and an ion source at the high-voltage end of said tube, the improvement comprising an inert insulating gas atmosphere filling said tank under superatmospheric pressure and a shield adjacent to said ion source near the high voltage end of said tube, said shield being fabricated of an electron-absorbing material for absorbing electrons streaming back toward the high-voltage end whereby the generation of X-rays and the consequent ionization of the gas are suppressed.

11. The apparatus of claim in which the gas contains as the only active ingredient a material that can trap free electrons.

12. In an electrostatic generator for positive-ion acceleration comprising a fluid-tight tank, an evacuated ionaccelerating tube in said tank, means for establishing a voltage gradient along said tube and an ion source at the high voltage end of said tube, the improvement com-- prising an inert insulating gas atmosphere fiiling said tank under superatmospheric pressure, said gas containing as its only active ingredient a material that tends to trap free electrons, means for absorbing electrons streaming back toward the high-voltage end and an X-ray absorber adjacent to said electron-absorbing means so arranged as to absorb X-rays generated therein, whereby the generation of X-rays in said tube and ion source and the consequent ionization of the gas are suppressed.

13. The apparatus of claim 12 in which the X-ray absorbing means is cup-shaped and encloses at least partially the electron-absorbing means.

14. In an electrostatic generator for positive-ion acceleration comprising a fluid-tight tank, an evacuated ionaccelerating tube in said tank, means for establishing a voltage gradient along said tube and an ion source at the high-voltage end of said tube, the improvement comprising an inert insulating gas atmosphere filling said tank under superatmospheric pressure, said gas containing as its only active ingredient a material that tends to trap free electrons, electron-absorbing means in said accelerating tube adjacent to said ion source and fabricated of an electrically conductive material comprising an element of atomic number lower than 13 arranged to absorb electrons streaming back toward the high-voltage end of said tube and an X-ray absorber between the electron absorbing means and said ion source whereby the generation of X-rays and the consequent ionization of the gas are suppressed.

15. The apparatus of claim 14 in which the electronabsorbing means comprises beryllium as the essential ingredient and the X-ray absorber comprises lead.

16. The apparatus of claim 14 in which the electronabsorbing means is composed of lithium hydride with an electrically conductive and corrosion resistant coating of aluminum.

17. In an electrostatic generator for positive-ion acceleration comprising a fluid-tight tank, an evacuated ionaccelerating tube in said tank, means for establishing a voltage gradient along said tube and an ion source at the high-voltage end of said tube, the improvement comprising means within said tube for absorbing electrons streaming back toward the high-voltage end whereby the generation of X-rays is suppressed.

18. The apparatus of claim 17 in which the electron absorbing means is a conductive plate positioned to shield the ion source from said accelerating tube and comprising an element of atomic number less than 13.

19. The apparatus of claim 17 in which the accelerating tube has annular electrodes and at the inner periphery of such electrodes there is an electron-absorbing material of atomic number lower than 13.

20. In an electrostatic generator for positive-ion acceleration comprising a fluid-tight tank, an ion-accelerating tube in said tank, means for establishing a voltage gradient along said tube and an ion source at the high-voltage end of said tube, the improvement comprising an inert insulating gas atmosphere filling said tank under superatmospheric pressure, a plate within said tube at the highvoltage end thereof adjacent to said ion source and fabricated of an electron-absorbing material comprising an element of atomic number lower than 13 to absorb electrons streaming back toward the high-voltage end and electrically conductive means connecting said plate with a source of high potential whereby the generation of X-rays and the consequent ionization of the gas are suppressed.

21. The apparatus of claim 20 in which the plate is contained in a cup-shaped X-ray shield situated between the plate and the ion source, the shield comprising an X-ray absorbing material.

References Cited in the file of this patent UNITED STATES PATENTS 2,578,908 Turner Dec. 18, 1951 2,611,878 Coleman Sept. 23, 1952 2,715,694 Yockey Aug. 16, 1955 

